Equol: A substance produced in the intestine as a metabolite of soybeans and soy foods. Equol is a non-steroidal estrogen that acts as an anti-androgen by blocking the hormone dihydrotestosterone. Some people are able to produce equol while others cannot.
Equol is chemically unique among the isoflavones (a family of phytoestrogens, plant estrogens). It is the major metabolite of the phytoestrogen daidzein, an isoflavone abundant in soybeans and soy foods.
Equol was so named because it was first isolated (1932) from the urine of pregnant mares and later (1982) was identified in the urine of humans consuming soy foods.